The present invention relates to a power transmitting device.
Power transmitting devices of known configuration include a type that has a cylindrical first rotor, which receives driving force and rotates, and a shaft-like second rotor, which is coaxial with and rotatably arranged in the first rotor. A clutch mechanism is located between the first rotor and the second rotor, and selectively permits and shuts off torque transmission between the first rotor and the second rotor. The power transmitting device is arranged in, for example, a power transmission path to auxiliary drive wheels of a four-wheel-drive vehicle and controls transmission of the power to the auxiliary drive wheels. The clutch mechanism is formed by an electromagnetic clutch operated by an electromagnet. Mainly to prevent decrease of transmitted torque caused by leakage of magnetic flux, a front housing member, which is the first rotor, is formed of non-magnetic material such as an aluminum alloy (see, for example, Japanese Laid-Open Patent Publication No. 10-292827).
Generally, a power transmitting device is rotatably supported by a ball bearing arranged outside the front housing member. To improve the wear resistance, the ball bearing has an inner race and an outer race that are formed of iron based material such as high-carbon chromium steel. The inner race and the outer race normally exhibit a thermal expansion coefficient lower than the thermal expansion coefficient of the front housing member. Thus, if the power transmitting device is heated by friction heat produced by the clutch mechanism, the front housing member thermally expands to deform and expand the inner race in a radially outward direction, reducing the size of the clearance in the ball bearing. This generates excessively great friction in the ball bearing, thus shortening the life of the ball bearing or causing a seizure of the ball bearing. To solve this problem, conventionally, the clearance in the ball bearing is set to a comparatively large size. However, preload needs to be applied to the ball bearing in order to suppress noise caused by such large-sized clearance. That is, the large-sized clearance in the ball bearing complicates the manufacturing steps and raises the costs. In this regard, the conventional power transmitting device is yet to be improved.